Audio output systems in which an audiovisual device such as a television receiver (below, abbreviated as “TV”), an external audio device such as an AV amplifier to which an external speaker is connected, and an audio reproduction device such as a DVD player or recorder, have been connected are becoming widespread. In such an audio output system, when audio is output from the TV, and when audio is output from the external speaker connected to the AV amplifier, the user is required to switch the volume and input/output path of each device, so operation is troublesome.
In order to reduce the trouble of operation, technology has been proposed in which the devices are connected with a communications line, and an ID number is given to the individual devices, so that the devices can be aware of each other (Patent Document 1). With this technology, by sending commands from the respective devices to another device, the other, connected device is controlled.    Patent Document 1: Japanese Patent No. 3382118